With the widespread adoption of lithium-ion batteries (LIBs), safety concerns associated with flammable organic electrolytes have become increasingly critical. Solid-state lithium batteries (SSLBs), with enhanced safety and higher energy density potential, are regarded as a promising next-generation energy storage technology. However, the practical application of solid-state electrolytes (SSEs) remains hindered by several challenges, including low Li+ ion conductivity, poor interfacial compatibility with electrodes, unfavorable mechanical properties and difficulties in scalable manufacturing. This review systematically examines recent progress in SSEs, including inorganic types (oxides, sulfides, halides), organic types (polymers, plastic crystals, poly(ionic liquids) (PILs)), and the emerging class of soft solid-state electrolytes (S3Es), especially those based on “rigid-flexible synergy” composites and “Li+-desolvation” mechanism using porous frameworks. Critical assessment reveals that single-component SSEs face inherent limitations that are difficult to be fully overcome through compositional and structural modification alone. In contrast, S3Es integrate the strength of complementary components to achieve a balanced and synergic enhancement in electrochemical properties (e.g., ionic conductivity and stability window), mechanical integrity, and processability, showing great promise as next-generation SSEs. Furthermore, the application-oriented challenges and emerging trends in S3E research are outlined, aiming to provide strategic insights into future development of high-performance SSEs.
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Review
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With the rapid advancement of portable electronic devices, electric vehicles, and large-scale energy storage grids, next-generation rechargeable batteries with higher energy densities and enhanced safety have attracted much attention. Solid-state lithium metal batteries with high-safety solid-state electrolytes and lithium metal at the maximum specific capacity as an anode hold a tremendous potential for the next generation of rechargeable batteries. As a critical component of solid-state lithium metal batteries, solid-state electrolytes play a pivotal role in advancing their development. The existing solid-state electrolyte materials (i.e., organic polymer electrolytes, oxide electrolytes, sulfide electrolytes, and halide electrolytes) are systematically investigated, with the oxide solid-state electrolytes standing out due to their excellent overall performance. To achieve the practical application of oxide solid-state electrolytes, substantial research efforts are directed at modifying ceramic grain boundaries to improve ionic conductivity and mechanical properties and reduce electronic conductivity. Also, research on ceramic films is equally important to achieve a high energy density.
The sintering process of ceramics is a critical factor affecting material properties, especially for the improvement of grain boundary. Relative density, grain boundary strength, and ionic/electronic conductivity are dominant factors impacting the long-term stability of the battery, with relative density as the most fundamental performance of ceramic electrolyte that enables other characteristics to be manifested effectively. The regulation of the preparation process, including powder preparation method, ceramic sintering temperature, and atmosphere, can effectively enhance the ceramic relative density. The powder preparation route, in particular, has a considerable effect on the final properties of LLZO ceramic, such as grain size and size distribution, relative density, microstructure, and ionic/electronic conductivity. In the sintering, some parameters such as sintering temperature, dwelling time, pressure, and the lithium oxide atmosphere play a crucial role in promoting the densification of LLZO ceramic.
Atmospheric sintering is still the most common, simple, and low-cost method to prepare LLZO ceramics although it has a longer sintering time and a higher sintering temperature, compared to the pressurized sintering. The prolonged high-temperature process leads to lithium loss and the formation of impurity phases like La2Zr2O7 in the electrolyte, which is detrimental to reducing the densification and ionic conduction behavior of the ceramics. To promote the densification and lower the sintering temperature, various kinds of sintering aids are adopted to assist the sintering process. A variety of low melting point additives, such as Li3BO3, LiO2–B2O3–SiO2–CaO–Al2O3, etc., are used as ceramic sintering aids. Subsequently, optimization of sintering processes and sintering aids are emerging, such as embedding-sintering, atmosphere-reversible control additives, lithium source-green body separation sintering, endogenous atmosphere sintering aids, etc.. LLZO ceramics sintered in the endogenous atmosphere with sintering additive of Li6Zr2O7 can realize the densities of up to 97.21% in the absence of mother powder and under atmospheric pressure.
In highly densified LLZO ceramics, dendrite growth and even piercing of the ceramic sheet are still inevitable, indicating that dendrite growth is related to ceramic relative density and ionic conductivity and is also closely related to the electronic conductivity and fracture toughness of ceramics. Multifunctional additives are reported to regulate the compositions, microstructures, as well as physical and electrochemical properties at the internal grain boundaries of LLZO ceramic in addition to improving the relative density and ionic conductivity of the ceramics. Among them, (Li2O)0.733(ZrO2)0.267, Li6Zr2O7, Li2WO4, Li2CuO2, LaTiO3, etc., are considered as effective sintering additives.
To realize the high energy density of solid-state lithium metal batteries, thin-filming of ceramic electrolyte has attracted recent attention. Since the reduction of the thickness of LLZO ceramic leads to a decrease in the mechanical properties of the ceramics, a balance between the thickness and strength of the electrolyte is of great significance. Realizing the thin-film of LLZO ceramic through rational structural design is crucial to achieving a high energy density of solid-state batteries. In this case, ceramic films with different structures, such as single-layer dense ceramic, multilayer ceramic, and single-layer porous ceramic, are reported. A lithium-metal battery based on an interface-optimized 74 μm-thick single-layer dense LLZO ceramic has the excellent long-cycle stability of more than 800 cycles. A lithium-symmetric battery based on an optimized 115 μm-thick multilayer ceramic exhibits an ultra-high critical current density of 100 mA·cm–2. A 12 μm-thick single-layer porous ceramic composited with a polymer electrolyte is expected to achieve energy densities of greater than 350 W·h·kg–1. In addition, the quality of the green body (i.e., uniformity, flatness, and residual stress) and the sintering regime have a significant effect on the phase purity, ionic conductivity, surface state, and film flatness of LLZO after sintering due to the high sintering temperature of LLZO as well as the volatile properties of Li, La, Zr, and O in the composition. Therefore, some novel preparation processes to prepare ceramic films are developed in recent years. In particular, the combination of tape-casting and ultrafast-sintering has a promising application in the preparation of LLZO ceramic films.
Although significant progress is made in the development of garnet-type solid-state electrolytes, their practical application in all-solid-state lithium batteries still faces challenges. Future research should focus on some key issues, i.e., the application of artificial intelligence and high-throughput computing in ceramic sintering additives, the development of new preparation methods for ceramic thin films, and the preparation, evaluation, and analysis of the full battery. Moreover, it is essential for the practical application of ceramic-based solid-state lithium metal batteries to simultaneously adapt testing standards, characterization techniques and failure analysis methods.
Solid-state lithium metal batteries are one of the most promising options for next-generation batteries pursuing high-energy density and high-safety. However, the inevitable volatilization of lithium compounds during sintering leads to low relative density and low ionic conductivity of solid-state electrolytes. Herein, the dynamic lithium-compensation mechanism is proposed to facilitate the densification of Ta-substituted garnet-type electrolyte (Li6.5La3Zr1.5Ta0.5O12 (LLZT)) through the reversible manipulating of Li2O atmosphere. Li2ZrO3 is used as mother powder additive, which reacts with Li2O in sintering atmosphere and forms Li6Zr2O7. Li2ZrO3/Li6Zr2O7 buffer pair manipulates the sintering Li2O atmosphere, which is vital for LLZT, within the Li2O partial pressure range corresponding to Li2ZrO3 and Li6Zr2O7. Furthermore, the reversibility mechanism of buffer pair for Li2O absorption and release is revealed. The obtained LLZT exhibits a relative density of over 96% and an ionic conductivity exceeding 7 × 10−4 S·cm−1 with no abnormal grain growth. The symmetric cell demonstrates an excellent lithium dendrite suppressing ability (stable cycling at a current density of 0.3 mA·cm−2 for over 1000 h). Such dynamic lithium-compensation strategy has been successfully applied in atmosphere manipulation of LLZT sintering process, which reduces the dependence of LLZT on the Li2O atmosphere, making it conducive to large-scale preparation of electrolyte ceramics.
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Research Article
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Solid-state sodium-ion batteries with sodium metal anodes possess high safety and reliability, which are considered as a promising candidate for the next generation of energy storage technology. However, poor electronic and ionic conductivities at the interface between electrodes and solid-state electrolytes restrict its practical application. Herein, we demonstrate a β″-Al2O3 electrolyte with a vertically porous-dense bilayer structure to solve this problem. The carbon-coated vertically porous layer serves as a high mass-loading host for Na3V2(PO4)3 cathode and provides fast electronically and ionically conductive pathways. In addition, the dense layer is produced to prevent sodium dendrite growth and improve mechanical strength of β″-Al2O3 electrolyte. Experimental results show that the cathode loading in vertically porous layer can reach to 8 mg cm−2, and the porous-dense bilayer β″-Al2O3 electrolyte-based battery exhibits a reversible specific capacity of 87 mAh g−1 and a capacity retention of 95.5% over 100 cycles at a current density of 0.1 C, which is superior to that of the traditional dense β″-Al2O3 electrolyte-based battery. This work based on electrolyte structure design represents an efficient strategy for the development of solid-state sodium-ion batteries with high mass-loading cathode.
Open Access
Research Article
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Cubic Li-Garnet Li7La3Zr2O12 (c-LLZO) is one of the promising solid-state electrolyte candidates for the next generation high safety solid-state batteries. However, preparing the electrolyte has many challenges. Li-loss during high temperature sintering is one of them. Mother powder with the same component of green garnet pellets is often used to co-fire with the pellets to compensate the Li-loss. Due to the high weight ratio of rare earth element La and non-recyclability of mother powder, it is worthy to explore low-cost mother powders to replace them. In this paper, low cost compounds such as Li5AlO4, Li2TiO3, Li2SiO3, Li4SiO4 and (Li2O)x-(ZrO2)1-x (x = 0.6–0.8) are investigated for substitution of the mother powder for compensating Li-loss during the sintering of LLZO. Dense Li6.4La3Zr1.4Ta0.6O12 (LLZTO) samples have been prepared by sintering with (Li2O)0.733(ZrO2)0.267 powder at 1150 ℃ for 5 h with the relative density of 95% and conductivity of 5.7 × 10−4 S cm−1 at 25 ℃, which show same performance with LLZTO ceramics sintered with LLZO mother powder.
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